1umlaut

: a mark ¨ placed over a vowel (such as a u in German) to indicate a specific pronunciation

Full Definition of UMLAUT

1

a: the change of a vowel (as \ü\ to \ē\ in goose, geese) that is caused by partial assimilation to a succeeding sound or that occurs as a reflex of the former presence of a succeeding sound which has been lost or altered

b: a vowel resulting from such partial assimilation

2

: a diacritical mark ¨ placed over a vowel to indicate a more central or front articulation — compare diaeresis